Cache contains only ascii in StringImpl

third_party/WebKit/Source/wtf/text/ASCIIFastPath.h

 /*
  * Copyright (C) 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 60 matching lines @@
 template <typename CharacterType>
 inline bool isAllASCII(MachineWord word) {
   return !(word & NonASCIIMask<sizeof(MachineWord), CharacterType>::value());
 }
 
 // Note: This function assume the input is likely all ASCII, and
 // does not leave early if it is not the case.
 template <typename CharacterType>
 inline bool charactersAreAllASCII(const CharacterType* characters,
                                   size_t length) {
+  DCHECK_GT(length, 0u);
   MachineWord allCharBits = 0;
   const CharacterType* end = characters + length;
 
   // Prologue: align the input.
   while (!isAlignedToMachineWord(characters) && characters != end) {
     allCharBits |= *characters;
     ++characters;
   }
 
   // Compare the values of CPU word size.
@@ skipping 82 matching lines @@
   for (size_t i = 0; i < length; ++i) {
     DCHECK(!(source[i] & 0xff00));
     destination[i] = static_cast<LChar>(source[i]);
   }
 #endif
 }
 
 }  // namespace WTF
 
 #endif  // ASCIIFastPath_h